The present invention relates to brine fluids which are viscosified with N-heterocyclic organophosphorus compounds.
During the drilling of an oil well, a usually aqueous fluid is injected into the well through the drill pipe and recirculated to the surface in the annular area between the well-bore wall and the drill string. The functions of the drilling fluid include: lubrication of the drill bit, transportation of cuttings to the surface, counterbalancing formation pressure to prevent an influx of oil, gas or water into the well, maintenance of hole stability until casings can be set, suspension of solids when the fluid is not being circulated, and minimizing fluid loss into and possible associated damage/instability to the formation through which drilling is taking place.
Proper counterbalancing of formation pressure is obtained by establishing fluid density at the desired level usually via the addition of barite (greater than or equal to 95% barium sulfate). Transportation of cuttings and their suspension when the fluid is not circulating is related to the fluid viscosity and thixotropy which depend on solids content and/or use of a polymer. Filter loss control is obtained also by the use of clays and/or added polymers.
Fluid properties are constantly monitored during the drilling operations and tailored to accommodate the nature of the formation stratum being encountered at the time. When drilling reaches the producing formation special concern is exercised. Preferentially low solids content fluids are used to minimize possible productivity loss by solids plugging. Proper fluid density for counterbalancing formation pressure may be obtained by using high salt concentration aqueous brines while viscosity and filter loss control may be obtained by polymer addition. Substantial future oil well drilling will take place at greater and greater depths where higher and higher temperatures are encountered. High temperatures coupled with the desire for low solids content and preferably no added solids, require brine tolerant and high temperature stable compounds for viscosity and filtration control. Conventionally employed polymers such as starch, carboxymethyl cellulose, and modified polyacrylates are not stable at high temperatures and some have severe brine tolerance limitations.
For very deep drilling in particular, say below approximately 15,000 feet, the highest density drilling fluids are required to counterbalance formation pressures so as to insure the integrity of drilled holes. At these depths, temperatures of approximately 150.degree. C. and higher are encountered. One of the densest, clear brine drilling fluids is based on ZnBr.sub.2 /CaBr.sub.2 concentrates (19 lbs./gal., approximately 56% ZnBr.sub.2 and 20% CaBr.sub.2). However, deep well operations with this system have been severely limited by the fact that no viscosifiers, fluid loss control agents, corrosion inhibitors, etc., have yet been developed able to withstand temperatures of 150.degree. C. and above without degradation or gellation at pH's approximately 2 to 5, characteristic of this type of fluid.